The efficiency of an axial-flow gas turbine is influenced, inter alia, by leakage flows of the compressed gas which occur between rotating and non-rotating components of the turbine. In this context, the gap which occurs between the ends of the rotor blades and the housing wall which surrounds the rotor blades has an influence on the magnitude of the leakage flows. The efficiency of the gas turbine can be increased by reducing the size of or sealing this gap. However, on account of the forces which occur when the turbine is operating and the thermal load, the size of the gap cannot be reduced arbitrarily. Various sealing mechanisms are known with a view to reducing the leakage flows in these regions.
For example, DE 198 48 103 A1 describes a sealing arrangement for reducing leakage flows within a rotary turbomachine, in which a felt-like material, which reduces the axial gap flows of the hot gas, is provided as sealing element between guide vanes and/or rotor blades and adjacent components of the turbomachine.
A technique which is frequently used to reduce the leakage flows in axial rotary machines is described in DE 198 21 365 A1. In this technique, sealing areas between the casing and the rotor are designed as what are known as labyrinth sealings. In these labyrinth sealings, one or more cutting elements are formed on the rotor or the rotor blade connected to this rotor and engage in counterpart surfaces on the casing wall during rotation. In this context, it has proven particularly advantageous for the counterpart surfaces to be designed in the form of honeycomb sealings.
A sealing arrangement which is designed as a labyrinth sealing and has counterpart surfaces of honeycomb structure of this type for a gas turbine is also explained in more detail in U.S. Pat. No. 5,967,745.
When a gas turbine having a sealing arrangement of this type is operated for the first time, however, the cutting elements on the rotor blades have to remove material from the counterpart surface in order to produce the intermeshing structure with a view to achieving the sealing action. Also, during operation the counterpart surface often comes into contact with the cutting elements, on account of the different forces and thermal deformation which occur at different loads. In this context, it must be ensured that the cutting elements of the rotor blades are not worn away excessively.
Therefore, it is attempted on the one hand to reduce this wear by selecting a material which is significantly harder than the counterpart surface to be used for the cutting elements of the rotor blades. On the other hand, it is also known, for example from U.S. Pat. No. 5, 952,110, to provide the cutting elements with an abrasive coating, in which abrasive particles protrude from the surface of the cutting element. These abrasive particles grind down the counterpart surface in the corresponding region.
The cutting element, which is generally formed by the shroud or the blade root of the rotor blades or is fitted thereon, engages in the counterpart surface in the above arrangements in order to achieve a sealing action. Axial and radial expansions when the turbomachine is operating cause the counterpart surface to be worn down in both the radial direction and the axial direction, during which process the cutting elements of the shroud or of the blade root of the rotor blades should not undergo any wear, in order not to disturb the balance of the rotor.
However, in the event of prolonged operation of gas turbine installations with sealing arrangements of this type, it has been found that the direction of the wear may be reversed in the event of unfavorable temperature condition or at certain circumferential velocities during operation, so that the rotating part becomes worn while the counterpart surface, particularly in the case of honeycomb structures, remains virtually untouched.